If the chemical reaction is written in words instead of a formula, you first need to convert it to the reaction format (i.e. A + B --> C + D). Once you have the reaction, you can then count how many you have of each atom type on the reactant side and on the product side. I always tell students to make a list of elements present and then count them up for both sides.
Now that you know what you have, you can figure out what you need to do to balance it. If you have 3 C on one side and 1 C on the other, you'll need to put a coefficient of 3 in front of the C so that you have the same number of C atoms on both sides. Repeat this until you have the same number of atoms on both sides.
A couple of things to watch out for
- If you have an element (usually O or H) in two compounds on the *same* side of the reaction, balance them last.
- If there is a subscript in the formula, remember to take that into account when counting the number of atoms (i.e. CaCl2 has 1 Ca atom and 2 Cl atoms).